This invention relates generally to the controlling of pen-to-print medium spacing on a wet ink printer. More particularly, the invention concerns an apparatus for adjusting the pen-to-print medium spacing automatically according to a selected thickness of a print medium.
Typically an ink-jet printer, or any printer using wet ink, include a pen (also called a printhead) and a print platen for supporting a print medium for printing with the pen. An area between the pen and the print platen is commonly known as a print zone of the printer. The print platen guides and supports the print medium in the print zone during printing. The printer also includes a print medium feed mechanism for feeding a print medium through the print zone. During printing, ink is placed on the print medium by dropping or ejecting the ink from the pen, or by any other printing method well-known by those skilled in the art. The quality of a printout on the print medium depends on the resolution of the printer. The resolution is defined as the number of drops of ink required to cover a given area. For example, a printer with a 600 dots-per-inch (dpi) resolution is able to print dots of a size of {fraction (1/600)} of an inch. To achieve higher resolution and thus higher quality printing, it is a constant goal to achieve even smaller dot sizes from the pen. In addition to dot size, it is crucial that the drop be placed accurately on a desired position on the print medium. Inaccuracy in placement will result in a printout that lacks sharpness. Also inaccurate placement of dots will affect the colors of a printout since the colors are obtained by a half-toning process. There are several factors affecting the accuracy of placement of ink drops. These factors include the control of the movement of the pen, the timing of firing pulses applied to the pen and other known factors. One factor affecting placement accuracy is the draft that is created by movement of the pen during printing. To reduce the effect of the draft, a print medium is brought as close to the pen as possible. The distance between the pen and the print platen supporting the print medium is known as the pen-to-print medium spacing or distance. The smaller the pen-to-print medium distance, the less likely printing is affected by the draft. However, there is a limit to this reduction of pen-to-print medium distance. When a print medium is brought too close to the pen, smearing will occur during printing. Ink used in wet ink type printing includes a relatively large amount of water. As the wet ink comes into contact with the print medium, the water in the ink saturate the fibers of the print medium, causing the fibers to expand, which in turn causes the print medium to buckle. Such buckling will cause the print medium to come into contact with the pen during printing. Therefore, some allowance is required to prevent such a buckling print medium from touching the pen. Typically in the production of such printers, the pen-to-print medium distance is calibrated for a commonly used media thickness of for example 0.1 mm. With this media thickness, the printer mechanism is adjusted such that a good quality printout is achieveable. Because of the requirement to support media of different thicknesses, some printers are provided with mechanical levers for a user to manually adjust the pen-to-print medium spacing. Usually two values of pen-to-print medium spacing are provided, one for thinner media and the other for thicker media. One disadvantage of such a system is that the quality of printing is contingent on the user remembering to move the lever to the correct position for a print medium. If the pen-to-print medium spacing is incorrectly set, poor quality printout will result. For example if high pen-to-print medium distance is selected for a thin medium, the earlier mentioned problem of draft will affect the accuracy of the placement of the ink drops. If low pen-to-print medium is selected for a thick medium, smearing may occur. It is therefore important that a user sets the lever to the correct position before commencing printing.
To overcome this problem of a user having to properly set the pen-to-print medium distance, some printers are designed to detect the widths of a print medium and to adjust the pen-to-print medium spacing accordingly. However, such a design is restrictive in the sense that it accepts only certain print media of the appropriate size and thickness.
From the foregoing, the prior art therefore has a need for an improved method and apparatus for adjusting pen-to-print medium spacing which is less error prone and which is able to accept media of different sizes and thicknesses.
In accordance with a preferred embodiment of the present invention, an apparatus for adjusting a pen-to-print medium spacing in a printer has a pen and a print platen. The print platen supports a print medium for printing using the pen. The apparatus also includes a datum for holding either the pen or the print platen a first predetermined pen-to-print medium spacing away from the other. The pen or print platen is resiliently biased against the datum. The pen or print platen can be moved away from the datum to define a gap therebetween. The apparatus further includes a bar which is moveable into and out of the gap. When the bar is in the gap, the pen or print platen rests against the bar to define a second predetermined pen-to-print medium spacing. The bar is preferably moved by a means which is activated independently of the size of a print medium.
Preferably the means for moving the bar includes a rocker lever which is pivoted to a support. The bar is attached to the rocker lever. The rocker lever can be tilted to an engaged position to place the bar between the datum and the pen or the print platen to define the second predetermined pen-to-print medium spacing. The bar-moving means also includes a gear which is slidably coupled to the rocker lever for rotating the rocker lever when the gear is rotated. This gear can be rotated independently of the rocker lever when the rocker lever is obstructed. Also included in the bar-moving means is an arm which is pivoted and resiliently biased to the rocker lever. The arm has a transverse guide pin. A first blocking tab extends in an opposite direction to the guide pin for holding the guide pin, the arm and the rocker lever to a first retracted position when the gear is rotated in one direction of rotation. A second blocking tab extends in the same direction as the first blocking tab for holding the guide pin, the arm and the rocker lever to a second retracted position when the gear is rotated in an opposite direction of rotation. The first and the second blocking tabs define a gap therebetween for allowing the exit of the guide pin. A third tab also extends in the same direction as the first and second blocking tabs. This third tab is positioned between the first and the second blocking tabs for defining a cyclic path for the guide pin to move between the first retracted position and the second retracted position when the gear is alternatingly rotated a predetermined angular distance in each direction of rotation. To change the pen-to-print medium spacing, the gear is rotated in a predetermined sequence to allow the guide pin to leave the cyclic path via the gap to tilt the rocker lever to the engaged position.
The apparatus allows the pen-to-print medium spacing to be adjusted independently of the size of a print medium. In order for the printer to carry out an adjustment, a parameter indicating the desired pen-to-print medium spacing is remotely sent to the printer.